Pliocene-Pleistocene HT-LP metamorphism during multiple granitic intrusions in the southern branch of the Larderello geothermal field (southern Tuscany, Italy)

This work presents the results of a multidiscipinary study carried out in the southern branch of the Larderello geothermal field (Travale-Montieri area), which was based on the integration of field data with information on the deeper structures as derived from interpretation of seismic reflection profiles and the P-T-t history of the metamorphic substratum as reconstructed from borehole data. Our data document that the structural and metamorphic signature of the metamorphic substratum is chiefly related to a prograde HT-LP metamorphic overprint, ranging in age from c. 2.8 to 0.7 Ma. The metamorphic climax was attained within the K-feldspar zone and equilibrated at P-T conditions of about 0.2 GPa and 650 °C. We interpret these new findings as evidence of Pliocene-Pleistocene thermal metamorphism associated with multiple granite intrusions at shallow levels in the crust. These results (1) add strength to the interpretations emphasizing the contribution of Neogene thermal metamorphism to the tectonometamorphic signature of the basement rocks found at depth in the Larderello geothermal field, and (2) provide insights on the thermal structures of the crustal section hosting the geothermal field and on the modes through which the long-lived positive thermal anomaly operated in southern Tuscany.